Important mechanisms underlying the control of feeding and fat deposition have been conserved between Drosophila melanogaster and human. This validates the fruit fly as a genetic model system for identifying and characterizing novel genes that mediate metabolic processes. In Preliminary Studies, we have demonstrated that serotonin (5-hydroxytryptamine, 5HT) acts as a potent inhibitor of fly feeding behavior through activation of one or more 5HT receptors. These findings provide a direct parallel with mammals, for which it has been established that the 5HT2C receptor is an important mediator of feeding behavior and a well recognized target for the treatment of obesity. Aim 1 of this application proposes to further explore the molecular mechanisms underlying the serotonergic control of feeding in Drosophila, utilizing a combination of existing mutants and RNA interference flies which are currently being generated in our laboratory. These strains will provide tools to investigate the extent to which different 5HT receptor subtypes as well as their tissue specific expression (e.g. CMS vs. intestine) underlie the serotonergic control of feeding. The highly sensitive food intake assay that we have established for assessing 5HT mediated food intake has also been utilized to begin a forward genetic screen (Aim 2). To date, 1315 strains of mutant Drosophila, each carrying a single precisely mapped insertion, have been screened for abnormalities in feeding. Twelve of the most promising candidates have been selected for detailed analysis (Aim 2A). To explore the role of the , candidate gene (and its corresponding mammalian homolog) in modulating feeding and/or fat deposition, a combination of approaches will be used, including biochemical (e.g. fatty acid profiling), histologic (e.g. scanning electron microscopy of fat tissue), and genetic (e.g. transgenic rescue) methodologies. Based on our initial success in identifying candidate genes, we propose to continue the forward genetic screen (Aim 2B). The goal of this subsequent effort is to define modifiers of (i) food intake under basal conditions (ii) food intake in the presence of 5HT (to identify modifiers of feeding within the 5HT and intersecting pathways) and (iii) fat deposition. To enhance the likelihood of finding a phenotype and of identifying a gene with physiologic relevance in humans, we will utilize bioinformatic tools to pre-select insertion strains for screening. The selection criteria will include (i) disruption of the transcription unit, and (ii) existence of a mammalian homolog corresponding to the targeted fly gene. To date, -2,500 insertion bearing strains which meet these criteria have been identified. Each of these strains will be assessed for alterations in feeding behavior and fat deposition. Outliers will be characterized as described above. We anticipate that the proposed studies will identify novel genes relevant to the pathophysiology of human obesity.